Active orders are open bids or asks waiting for a counter bid or ask. In other words they are orders waiting to be bought or sold by someone else (trade). Active orders are often placed through limit orders.
In the case where a limit order can't be fully filled due to inefficient liquidity on the orderbook the amount unfilled in the limit order will automatically be placed on orderbook as an active order.
All active orders/unfilled limit orders can be viewed in your 'My Active Orders' panel center bottom of the screen. Any order unfilled can be canceled or the whole batch can be canceled at once.